The present invention relates to an apparatus for visualising a body, by detecting the radiation of a tracer contained in this body and consisting of positron emitters. This apparatus can be used particularly in tomography and hence for the visualisation of an organ section, in a given sectional plane.
The principle of the operation of this apparatus is based on the measurement of the radiation emitted by a tracer injected into the body or organ which is to be examined.
It is known to obtain a gammagraphic image of a body containing a tracer, by means of a camera sensitive to gamma radiation. This camera has an optical system consisting of a collimator which selects only those gamma rays whose trajectories are perpendicular to the plane of observation; the other radiation is lost and the effectiveness of such a camera is therefore greatly reduced. Moreover, visualisation of the body results from the superimposing of sections through planes perpendicular to the direction of collimation.
A camera is also known which operates on the basis of positron detection and is known under the name of the positron Gamma camera of the "coordinate reconstituting" type. In this case, a tracer consisting of positron emitters is injected into the body which is to be visualised. The positrons dematerialise almost instantaneously into two gamma radiations emitted in two opposite directions. The geometric locus of the point of emission of these two sets of radiation is therefore a straight line which is determined by two detectors. The numbers of these two detectors are recorded; by using a computer to process this information, the coordinates of the point of emission can be reconstituted. This type of camera has the advantage of avoiding the need for collimation means and thus considerably increasing the number of items of information available compared with the camera mentioned hereinbefore. In these positron gamma-cameras, the number of disintegrations registered in a specific time as coming from one point is a function of the density of the tracer at this point, thus making it possible to build up a picture of the distribution of the tracer.
However, in these positron gamma-cameras of the coordinate-reconstituting type, the detectors used are generally detectors using sodium iodide doped with thallium. These detectors are very effective, but they do not give acceptable time resolutions for measuring the travel times.
It is also known to produce a positron gamma-camera using only measurements of travel times, i.e. a camera wherein the point of emission of the gamma rays is located, on the straight line joining the two detectors, by measuring the difference between the travel times of the two sets of radiation. With this type of camera, better time resolution can be obtained, thanks to the use of plastic scintillators. However, localisation thus achieved, solely by measuring the travel times, is not sufficiently precise, unless the efficiency of the system is unreasonably sacrificed. To obtain an image the quality of which was equivalent to that of conventional positron cameras, the measuring times would have to be increased within limits which are unacceptable in the gamma photography of the human body. In fact, the shorter the measuring time, the less time it takes to treat the patient, thus making it possible to use tracers with a very short life, which are greatly preferable as regards the patient's health when an organ or part of the body is to be tomographed or visualised.